D
Dave
- Jan 1, 1970
- 0
Awhile back I posted about an amp I am trying to repair. The amp is
composed of a +/- rectified and filtered power supply, an LM1036N
volume/tone/balance IC (two-channel), and a pair of LM1875T 20W TO-220 amps
which use bipolar power, no output filter caps. A bunch of various
resistors and caps and diodes. Very simple. I tested the resistors and
diodes in circuit, they look to be okay. I have no reason to suspect the
caps although I did replace one which appeared to be open circuit.
The symptoms: No sound on the right channel. When I turn the volume up
quickly it makes a "thump" on the bad channel. So. I started at the
outputs and worked backwards. First off, I bridged the inputs on the
LM1875's with a 1uF cap. Voila! Sound. So, the speaker works, the amp
works.
Another poster suggested that I use an analog meter to measure input and
output voltages of the LM1036N IC. So last night I soldered on some test
leads. I measured AC and DC voltages, and they varied a bit from channel to
channel, maybe 5V on one side and 4.2 on the other, no catastrophic failure
there. As an aside I kind of expected my analog multimeter to look like a
VU meter, moving up and down to the music, but didn't see that at all, very
steady. Then I noticed something: when I measured the potential between
the bad channel's INPUT to the LM1036 and ground, I heard a scratchy sound
come out of the bad speaker. Hmm. As the meter is basically a big resistor
in parallel with the circuit, I figured I'd try a smaller resisiter. I
changed the scale of my meter from 30V to 10V. Now I get tinny sound.
Changed to 3V. Meter was off-scale but I now had pretty good sound. Didn't
have the balls to try shorting the two, I have been burnt (actually my
components have been burnt) by trying dumb-ass things like that using the
axiom "if a little is good, a lot must be REAL good".
I have no formal training in electronics, but have done a lot of reading on
the good ol' internet and am keen to learn more. I am completely perplexed
as to what is happening. The only explanation I can conjure up (and sorry
if it sounds stupid but...) is maybe an excess of DC current on my signal
input which would screw up the bias of the transisters in the mixer IC.
There is a small electrolytic in series with the mixer output and the amp
input, plus a bunch of smaller ceramics from the signal trace to ground so I
don't think DC can be getting to the amp itself... and if it were my little
experiement with the bridging cap, above, wouldn't have worked so well.
I have another LM1036 mixer IC, but don't want to replace something that's
not broken.
Does anybody have any ideas? How can bridging my input signal to ground
with a resistor make sound come out? I've looked at this circuit long
enough that I can probably draw up a schematic if I can find a few hours.
THanks
Dave
composed of a +/- rectified and filtered power supply, an LM1036N
volume/tone/balance IC (two-channel), and a pair of LM1875T 20W TO-220 amps
which use bipolar power, no output filter caps. A bunch of various
resistors and caps and diodes. Very simple. I tested the resistors and
diodes in circuit, they look to be okay. I have no reason to suspect the
caps although I did replace one which appeared to be open circuit.
The symptoms: No sound on the right channel. When I turn the volume up
quickly it makes a "thump" on the bad channel. So. I started at the
outputs and worked backwards. First off, I bridged the inputs on the
LM1875's with a 1uF cap. Voila! Sound. So, the speaker works, the amp
works.
Another poster suggested that I use an analog meter to measure input and
output voltages of the LM1036N IC. So last night I soldered on some test
leads. I measured AC and DC voltages, and they varied a bit from channel to
channel, maybe 5V on one side and 4.2 on the other, no catastrophic failure
there. As an aside I kind of expected my analog multimeter to look like a
VU meter, moving up and down to the music, but didn't see that at all, very
steady. Then I noticed something: when I measured the potential between
the bad channel's INPUT to the LM1036 and ground, I heard a scratchy sound
come out of the bad speaker. Hmm. As the meter is basically a big resistor
in parallel with the circuit, I figured I'd try a smaller resisiter. I
changed the scale of my meter from 30V to 10V. Now I get tinny sound.
Changed to 3V. Meter was off-scale but I now had pretty good sound. Didn't
have the balls to try shorting the two, I have been burnt (actually my
components have been burnt) by trying dumb-ass things like that using the
axiom "if a little is good, a lot must be REAL good".
I have no formal training in electronics, but have done a lot of reading on
the good ol' internet and am keen to learn more. I am completely perplexed
as to what is happening. The only explanation I can conjure up (and sorry
if it sounds stupid but...) is maybe an excess of DC current on my signal
input which would screw up the bias of the transisters in the mixer IC.
There is a small electrolytic in series with the mixer output and the amp
input, plus a bunch of smaller ceramics from the signal trace to ground so I
don't think DC can be getting to the amp itself... and if it were my little
experiement with the bridging cap, above, wouldn't have worked so well.
I have another LM1036 mixer IC, but don't want to replace something that's
not broken.
Does anybody have any ideas? How can bridging my input signal to ground
with a resistor make sound come out? I've looked at this circuit long
enough that I can probably draw up a schematic if I can find a few hours.
THanks
Dave